The 3rd and 7th term of an arithmetic progression are -9 and 11 respectively. What is the 15th term?
$$T_{3}$$ = a + 2d = -9-------(1)
$$T_{7}$$ = a + 6d = 11-------(2)
on solving (1) AND (2)
d = 5 & a = -19
$$T_{15}$$ = a + 14d = -19 + 14(5) = 51
so the answer is option C.
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